Few know that Magento out of the box doesn't actually cache anything other than the Navigation and Footer blocks, which are basically static as it is. This module enhances performance by allowing developers to cache any block they want via simple layout xml changes - per layout handle. There is also support for disabling cache for specific handles such as checkout, etc, where you don't want to load the main content block from cache. ESI is supported in conjunction with Phoenix_VarnishCache, and allows for super-fast page views. Simply add esi="1" to any block XML definition, and the rest is automatic. A good block cache is vital for scaling a site, be sure to implement it before residing to full page cache.